1. Field of the Invention
This invention relates to a semiconductor device in which the lifetime of carriers can be controlled in a substrate and to a method of manufacturing the same.
2. Description of the Related Art
A proton irradiation technique is known as a technique of controlling the lifetime of carriers locally by irradiating charged particles. In this technique, protons are injected into a semiconductor substrate, forming a large number of crystal defects in a proton stopping region (in the semiconductor substrate). The crystal defects trap carriers, thus shortening the lifetime of carriers in a limited area of the substrate. The depth of the proton stopping region from a surface of the substrate can be controlled by changing acceleration energy of the protons. As a result, as shown in FIG. 1, a density of crystal defects at a deep level, for example, at a level 0.39 eV lower than a conductive band Ec can be controlled locally. The region I in FIG. 1 is the proton stopping region. Thus, the region I having a large number of the crystal defects can be formed in a desired local area in the substrate.
FIG. 2 is a cross sectional view showing an arrangement of a conventional high speed switching diode. By virtue of the above-mentioned proton irradiation technique, a high speed switching device which can be turned on by a low voltage, for example, can be obtained. As shown in FIG. 2, N-impurities are injected in a high concentration into an N.sup.+ -silicon layer 31, on which an N.sup.- -epitaxial layer 32 of a low concentration is formed. Further, a P.sup.+ -region 33 of a high concentration is formed on a portion of the surface region of the epitaxial layer 32.
The diode is turned on when a forward voltage is applied thereto, and turned off when a reverse voltage is applied thereto. When the diode is in an OFF state, a depletion region 34 is formed in a region near the PN junction between the regions 32 and 33, as indicated by a dotted line in FIG. 2. Carriers remain in the depletion region 34 at a beginning of an OFF period. Because of the presence of the carriers, the switching operation of the diode is made slow. To prevent this slow operation, the protons are injected by the proton irradiation technique into the region near the PN junction, in which the depletion region 34 is formed, thereby forming a trap region 35 having a great number of the crystal defects. The carriers remaining in the depletion region 34 at the beginning of the OFF period are trapped in the trap region 35. In general, a crystal defect region can be formed in a limited area of the substrate by injecting the protons and therefore a resistance component of the switching device is not influenced substantially by the injection.
As described above, the switching device which can be turned on at high speed by a low voltage can be obtained by forming a great number of crystal defects locally in the substrate at a predetermined depth from the surface of the substrate, thereby controlling the lifetime of carriers. This technique can be applied to various devices having a PN junction, such as transistor, thyristor, and the like.
An acceleration voltage and a dosage of the protons i.e., an amount of the protons irradiating the substrate are given as basic parameters of the technique of controlling the lifetime of the carriers by means of proton irradiation. The proton stopping region in a silicon layer is determined by the acceleration voltage, and the crystal defect density in the proton stopping region is determined by the dosage of the protons. Hence, when the substrate surface is irradiated once by the protons with the fixed acceleration voltage and the dosage, only one crystal defect region I is formed, as shown in FIG. 1. Accordingly, two or more crystal defect regions have to be formed to control the lifetime of carriers in a device that has a number of PN junctions at different depths in the substrate. It is necessary to irradiate the substrate of such device by the protons with various accelerating voltages in the same number of times as that of the crystal defect regions required. However, if the protons are irradiated repeatedly, the manufacturing steps are increased, resulting in high manufacturing cost and low productivity.